In one type of digital communication system, the subscriber stations are connected over a continuous loop facility to minimize the wiring between the subscriber stations and a common controller of the system. In such a digital loop communication system, the controller establishes a time division multiple (TDM) communication format where each signal frame includes multiple communication channels (time slots) as well as framing said control bits. Each subscriber station is synchronized to the TDM signal and appropriately receives and transmits signals in the correct communication channel or time slot of the TDM system. Such a signal is shown in U.S. Pat. No. 4,048,449, issued on Sept. 13, 1977 to Roderick Natebusch, which patent is hereby incorporated by reference herein as if reproduced in full.
The Natebusch patent discloses a conferencing call method for a time division multiplex (TDM) pulse code modulation (PCM) closed loop digital communication system. In that system, each subscriber station is connected to the digital loop by an interface circuit which operates to remove that conferees signal sample from the received loop signal to yield the sum of the signals from the other conferees. The outgoing signal is generated by adding that conferee's new signal sample to the existing sum of the signals from the other conferees. Additionally, that conferee's signal sample is stored by the interface for one frame cycle and subtracted from the received loop signal during the next frame to form that conferee's received signal.
It is also known in the art to connect multiple central office lines to such a digital loop communication system using standard TDM techniques. In such an arrangement, the signals from each central office line is digitally encoded and multiplexed as a separate communication channel or word in the TDM data frame. A conference call between a subscriber on one of these central office lines and subscribers on the digital loop system utilizes the communication channel assigned to the central office line for conversation transmission between the conferees. The voice conversation signals from the subscriber on the central office line are PCM encoded and handled by the digital loop system in the same manner as any loop subscriber signals. Thus, the signals from the central office line are appropriately added to and/or subtracted from the loop signal in the communication channel associated with that central office line.
However, when it is desired to establish a conference call including subscribers of two separate central office (CO) lines and subscribers of the digital loop system a problem arises because subscribers of one CO line communicate over a TDM channel which is different from the TDM channel that subscribers of the second CO line communicate. Thus, to assure that all the conference subscribers (conferees) receive the proper conference signal, the signals of one TDM channel (time slot) must be coupled to the second TDM channel and vice versa.